Personality (or axis II) disorders are common, disabling and highly co-morbid with important axis I psychiatric disorders such as major depression and anxiety disorders as well as alcohol abuse/dependence. Personality disorders (PDs) aggregate in families, and existing twin studies suggest this familial aggregation is due partly to genetic factors. However these were based on modest-sized clinical samples or self-report questionnaires and have not been confirmed by an interview-based epidemiologic twin study of PDs. The goal of this application is to conduct detailed analysis of a population-based study of young adult twins in Norway assessed using personal interviews for the lifetime history of both major axis I psychiatric and substance use disorders and all axis II disorders. Data collection -- supported from Norwegian sources --is now 94% complete, and will be finished by 9/30/03 resulting in 1,400 complete twin pairs. We will conduct an extensive series of analyses on this rich data set using both standard and recently developed statistical tools. Aims include: i) examine and correct for attrition effects in the registry; ii) apply both factor and IRT models to generate empirical PD scales; iii) conducting univariate twin analyses to clarify the etiologic role of genetic, shared-environmental and individual specific environment factors for personality disorder traits (PDTs); iv) performing multivariate twin analyses to understand the sources of covariation between both individual PDTs and PD clusters; v) examining a series of theory-driven bivariate twin analyses to determine the degree to which selected pairs of axis I disorders and PDTs reflect the similar or distinct genetic and environmental risk factors such as Major Depression (MD) and Depressive and Borderline PD, Social phobia and Avoidant PD, and Alcohol Abuse/Dependence and Antisocial PD; vi) examine etiologic links between PDs and prospectively collected data on pregnancy and birth complications and vii) attempt to replicate key findings emerging from an on-going study of axis I Disorders in the Virginia Twin Registry. This study has the capacity to address several key and largely unanswered empirical questions about PDs, the answers to which will be critical for proposed revisions in DSM-V.